Print head assemblies typically include a print head, one or more ink paths for supplying ink to the print head, and drive circuitry for driving the print head. During the operation of conventional print head assemblies, there has been observed a phenomenon where bubbles form on the surfaces of the ink paths and in the print head as ink flows therethrough. The formation of such bubbles causes blockages in the ink flow, reduces the wettability of the surfaces, and degrades print quality.
To ameliorate this problem, the surfaces of a print head assembly may be activated with a plasma species during or after fabrication. Plasma activation of the internal surfaces of the print head assembly renders these surfaces more hydrophilic and increases their wettability; this in turn inhibits bubble formation.
The hydrophilic characteristics conveyed to surfaces by plasma activation, however, degrade or relax with time. In the case of print head assemblies, one solution for ameliorating this problem is to prime the print head assemblies with ink, or an ink like fluid, after the surfaces thereof have been plasma activated, and to ensure that the print head assemblies remain primed with ink (or the ink like fluid) until they are used. Keeping a print head assembly primed with ink, or an ink like fluid, from the time of production until the time of use, however, introduces significant complexities, including the storage and transport of such primed print head assemblies.
Another solution for retarding the relaxation of plasma activated surfaces is to treat the activated surfaces with a polyethyleneimine (PEI) solution. According to the current understanding in the art, PEI relies predominantly on the formation of intramolecular carboxyl bonds with the activated surfaces. PEI is therefore less effective when used on surfaces activated with a plasma other than a carbon dioxide plasma.
U.S. Pat. No. 5,700,559, U.S. Pat. No. 5,807,636, and U.S. Pat. No. 5,837,377 describe a hydrophilic article for use in aqueous environments, including a substrate, an ionic polymeric layer on said substrate, and a disordered polyelectrolyte coating ionically bonded to said polymeric layer.
The plasma activation of a print head assembly is conventionally performed using a vacuum plasma processing method. Vacuum plasma processing methods, however, are expensive and time consuming. A vacuum plasma processing method requires costly and specialised equipment to create a vacuum and to generate a plasma within the vacuum. Further, significant time is required for loading and unloading a work piece into/from a vacuum chamber, creating and releasing the vacuum, and allowing the plasma to diffused through and activate the work piece.
A further disadvantage associated with vacuum plasma processing is that vacuum plasma processing is indiscriminate insofar as which surfaces of the work piece are activated, and to what extent they are activated. Directed activation of specific surfaces is generally difficult to achieve and the selective activation of internal surfaces alone is impossible.
Still further, the vacuum plasma processing method does not compliment serial/assembly-line type production process commonly used in the fabrication of print head assemblies. To enable the vacuum plasma process to be cost feasible, print head assemblies are processed in batches. The collation and later de-collation of print head assemblies into batches for vacuum plasma processing interrupts the work flow of serial, assembly-line type production processes and reduces the efficiency of the production process.
Quality control issues also arise from the discontinuity caused by the batch processing of print head assemblies for vacuum plasma processing. A first print head assembly removed from a vacuum processing batch and a last print head assembly removed from the same batch vary in age. For example, a print head assembly removed first from the batch exiting the vacuum plasma process has an active surface that is “younger” than a print head assembly removed last from the same batch. Such differences in age affect the results of further processing steps performed downstream of the vacuum plasma processing step.